Paper or paperboard normally is made by producing a stock slurry or furnish, comprised mainly of cellulosic wood fibers but also often containing inorganic mineral fillers or pigments, depositing the slurry on a moving papermaking wire or fabric, and forming a sheet from the solid components by draining the water. This process is followed by pressing and drying operations. Many different organic and inorganic chemicals are often added to the furnish before the sheet forming process in order to make processing less costly or more rapid, or to attain special functional properties in the final paper or paperboard product.
The paper industry continuously strives for improvements in paper quality as well as reductions in manufacturing costs. Sheet strength is often a key factor in achieving or balancing these goals. Increases in the strength potential of the fiber furnish, for example, enable the papermaker to improve sheet opacity and printability or reduce fiber furnish raw material cost through substitution of expensive fiber with elevated loadings of low cost filler. A stronger sheet also provides the opportunity for cost savings through a reduction in pulp refining energy.
Starches are used by the paper industry to increase the inter-fiber bond strength of paper or paperboard as typically characterized by standardized Tensile, Mullen Burst, or Scott Bond tests. Papermaking starches function to enhance the fiber furnish strength potential by creating additional hydrogen bonding sites between contiguous fiber surfaces when the sheet is formed and dried. Higher starch addition rates are often desired to achieve increases in bonding strength. However, starch adsorption on the fiber is generally incomplete, resulting in reduced starch efficiency, operating difficulties attributable to high levels of unadsorbed starch recirculating in the process filtrate circuit, and the resulting inability to further increase the starch addition level. These effects are evident even for the cationically derivatized starch products, which are utilized in an attempt to obtain greater adsorption.